The equipment necessary for refining operations is one of the major sources of costs in a refinery. The equipment can include catalytic reactors, fractionators and/or separators, and other supporting equipment. In a conventional process train, each catalytic reactor can have a dedicated fractionator or separator associated with the reactor, to separate out the various products produced in the catalytic reaction stage.
In diesel hydroprocessing, it is sometimes beneficial to include a dewaxing stage as part of reaction train in order to improve properties of the resulting diesel fuel such as pour point or cloud point. For feeds with suitably low levels of sulfur and/or nitrogen, the dewaxing stage can be included as part of a reactor that contains a hydrotreating stage. Feeds with higher levels of sulfur and/or nitrogen, however, may benefit from having a separate reactor for the dewaxing stage. This leads to a further increase in the capital costs necessary for generating a diesel fuel product.
U.S. Pat. No. 3,431,194 describes a process for lowering the pour point of a middle distillate feed. The middle distillate is split into a lower boiling fraction and a higher boiling fraction. The higher boiling fraction is hydroisomerized to reduce the cloud point and pour point of the higher boiling fraction. The higher boiling fraction is then recombined with the lower boiling fraction to form a middle distillate with improved pour point and cloud point relative to the original feed. Based on the examples, it appears that the cut point for the lower boiling fraction is selected so that the cloud point of the lower fraction is lower than the cloud point for the hydroisomerized higher boiling fraction.
U.S. Pat. No. 3,412,016 shows an example of a fractionator that includes multiple volumes. In U.S. Pat. No. 3,412,016, two independent refinery gasoline streams (such as a low octane and a high octane gasoline) are fractionated in the fractionator. The outputs from the fractionator are a light fraction and distinct heavy fractions from the two separate volumes in the fractionator. In the fractionator, the lighter portions of the two gasoline fractions are allowed to mix.
European patent publication EP 0819752 appears to provide another example of using a fractionator having multiple volumes. In EP 0819752, it appears that two separate input streams are provided to the fractionator. The vapor portions produced in each side of the fractionator are allowed to mix, leading to production of one or more light product fractions from the fractionator. Each side of the fractionator also produces a bottoms portion. In some figures, the bottom portions appear to remain separated after leaving the fractionator, while in other figures, the input to the second side of the fractionator includes portions of the bottoms from both sides of the fractionator.
U.S. Published Patent Application 2011/0132803 describes a two-stage hydroprocessing system that includes a divided wall column fractionator. Methods are described for using the hydroprocessing system to generate a plurality of distillate boiling range products from the divided volumes in the fractionator. Additionally, a naphtha and/or a kerosene product are generated from a common volume at the top of the fractionator.